The INSIDER Summary:

Because people tend to use the middle stalls, it doesn't necessarily mean the end stalls are any cleaner, though.

You're driving down the turnpike. That painful pressure below your navel has grown too intense to ignore, but you've got another hour and a half left to go before you get home. A blue sign flashes through your headlights, a rest stop. You pull in, walk to the public bathroom, and wrinkle your nose. A row of stalls stretches from wall to wall. Which should you choose?

Most of us enter this situation armed with nothing but our eyes, noses, and intuitions. But science can help. Research suggests you should avoid the middle stalls at all costs.

Here's why.

A wealth of research shows that, given several equally good (or gross) options, people tend to choose the middle one. Psychologists call this "centrality preference." That means that most people who entered the bathroom before you probably went for the center stalls and avoided the sides. With any luck, that could leave the side stalls a bit less nasty.

Skeptical? Psychologists have shown that centrality preference applies to public bathrooms.

In a paper published in 1995 in the journal Psychological Science, the psychologist Nicholas Christenfeld presented a series of short experiments in centrality preference. For one of them, he examined the habits of beachgoers in a coastal Californian public washroom.

It would have been at the very least impolite to stand around in there all day with a notebook recording which stalls people used. So Christenfeld came up with a useful proxy: toilet paper use. He enlisted the help of the bathroom's custodian, who kept track of how often toilet paper needed changing in each of the four stalls for 10 weeks.

The results: Far more people used the middle stalls than random chance would predict — 60% of finished rolls came from the central stalls, with only 40% from the end stalls.

An important caveat here is that just because fewer people use an end stall, that doesn't necessarily mean it's cleaner. Maybe the sort of people who use end stalls also make more of a mess. Maybe custodians know that outer stalls see less traffic and clean them less often. In any given bathroom, an unseen cue or quirk of design may shift people toward outer stalls.

But still, at my next rest stop I'll be locking the door as close to the wall as possible.